Which of the following statements is a tautology?

If the direction cosines of two lines satisfy the equations $l+m+n=0$ and $2lm+2ln-mn=0$,then the acute angle between those two lines is

An open-ended $U$-tube of uniform cross-sectional area contains water (density $1.0 \, g/cm^3$) standing initially $20 \, cm$ from the bottom in each arm. An immiscible liquid of density $4.0 \, g/cm^3$ is added to one arm until a layer $5 \, cm$ high forms,as shown in the figure. What is the ratio $h_2/h_1$ of the heights of the liquid in the two arms?

The rate constant of a first order reaction at $27^{\circ} C$ is $10^{-3} \ min^{-1}$. The temperature coefficient of this reaction is $2$. What is the rate constant (in $min^{-1}$) at $17^{\circ} C$ for this reaction?

The number of moles of electrons required to deposit $36 \ g$ of $Al$ from an aqueous solution of $Al(NO_3)_3$ is (At. wt. of $Al = 27$)

What is the optimal soil $pH$ for plant growth?